1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a semiconductor device comprising high-quality epitaxial layers which are grown on a single-crystal semiconductor substrate by molecular beam epitaxy (hereinafter abbreviated as MBE).
2. Description of the Prior Art
In recent years, the MBE growth technique has been remarkably developed, so that it is possible to control the formation of extremely thin epitaxial layers having a thickness of approximately 10 .ANG. or less that is the order of a monolayer thickness. This progress in the MBE growth technique has realized the manufacture of novel semiconductor devices utilizing a new effect based on a device structure having extremely thin layers, which could not be manufactured by conventional growth techniques such as liquid phase epitaxy (hereinafter abbreviated as LPE) or the like.
A typical example of such semiconductor devices is a GaAs/AIGaAs quantum well (QW) laser. In this GaAs/AlGaAs QW laser, an active layer has a thickness of about 100 .ANG. or less, resulting in the formation of quantum levels therein, while the active layer of the conventional double-heterostructure (DH) laser has a thickness of several hundreds of angstroms or more. This GaAs/AlGaAs QW laser is advantageous over the conventional DH laser in that it can attain a lower threshold current, good temperature characteristics, and excellent transient characteristics (see, for example, W.T. Tsang, Applied Physics Letters, Vol. 39, No. 10, pp. 786, 1981; N.K. Dutta, Journal of Applied Physics, Vol. 53, No. 11, pp. 7211, 1982; and H. Iwamura, T. Saku, T. Ishibashi, K. Otsuka, Y. Horikoshi, Electronics Letters, Vol. 19, No. 5, pp. 180, 1983).
Another typical example of the semiconductor device produced by MBE is a field effect transistor (FET) utilizing the high mobility characteristics of two-dimensional electron gas generated at an interface between GaAs layer and AlGaAs layer (see, T. Mimura et al., Japanese Journal Applied Physics, Vol. 19, p. L225, 1980).
By the use of the MBE growth technique, it is easy to produce semiconductor devices having novel compound semiconductor materials which cannot be grown by the conventional techniques such as LPE or the like. A typical example of such materials is AlGaInP which is used as a semiconductor crystal for a visible-light emitting semiconductor laser device. The MBE growth technique has realized the excellent crystal growth of AlGaInP, which was difficult to do by LPE or other conventional techniques. The semiconductor device obtained by growing the AlGaInP crystal by MBE can attain continuous laser oscillation with a wavelength of 670 nm at room temperature (see, for example, T. Hayakawa, Japanese Journal Applied Physics, Vol. 27, pp. L1553, 1988).
Such semiconductor materials that are difficult to grow on the semiconductor substrate without using MBE include, other than the above-mentioned AlGaInP, AlGaInAs used in a light-emitting communication device having a long oscillation wavelength of a 1.3 to 1.5 .mu.m band, or the like. Furthermore, a smooth heterointerface of the grown layers can be obtained by using the MBE growth technique.
Therefore, MBE allows the semiconductor devices utilizing quantum effects such as the QW lasers, HEMTs (high electron mobility transistors), or the like to be manufactured with ease.
In general, the above-discussed semiconductor devices have been formed on a (100)-oriented GaAs substrate. In an AlGaInP crystal which was grown on the (100)-oriented GaAs substrate by metalorganic chemical vapor deposition (MOCVD) or MBE, a bandgap of the resulting AlGaInP crystal layer was smaller than that expected from the conventional theories and experiments. This is because the group III atoms such as Al, Ga and In were not arranged at random in the AlGaInP crystal layer, but arranged to form a natural superlattice in the AlGaInP crystal (see, for example, A. Gomyo, Applied Physics Letters, Vol. 50, pp. 673, 1987; and O. Ueda, Japanese journal Applied Physics, Vol. 26, L.1824, 1987).
Actually, due to the formation of the natural superlattice, the oscillation wavelength of the semiconductor laser device with a GaInP active layer, although it was expected to be 650 nm, was as long as 670 nm.
On the contrary, when the above-mentioned AlGaInP crystal was grown on a (111)-oriented GaAs substrate, the oscillation wavelength of the resulting semiconductor laser device was not longer than that expected from the conventional theories and experiments.
Therefore, studies on semiconductor devices comprising the (111)-oriented GaAs substrates have become extensive rather than on semiconductor devices with the (100)-oriented GaAs substrates (see, for example, S. Yasuami, Applied Physics Letters; and M. Ikeda, ELECTRONICS LETTERS, Vol. 24, pp. 1094, 1988).
Also, it is disclosed by Hayakawa et al., that two-dimensional carrier confining effects (quantum size effects) which are generated when using extremely thin layers occurred more strongly in the semiconductor devices with the (111)-oriented GaAs substrates than in the semiconductor devices with the (100)-oriented GaAs substrates (see, for example, T. Hayakawa, Applied Physics Letters, Vol. 52, p. 339, 1988; and T. Hayakawa, Phys. Rev. B Vol. 60, p. 349, 1988).
Furthermore, it is disclosed that the reliability of the semiconductor laser devices formed on the (111)-oriented GaAs substrates by MBE is superior to that of the semiconductor laser devices formed on the (100)-oriented GaAs substrates by MBE (see, for example, T. Hayakawa, Japanese Journal Applied Physics, Vol. 27, p. L889, 1988).
In this way, in recent years, the semiconductor devices having crystal layers grown on the (111)-oriented GaAs substrates have been developed.
However, the above-discussed semiconductor device formed on the (111)-oriented substrate has a problem such that the crystal quality of the crystal layers grown on the (111)-oriented substrate is poor. For example, (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P (x=0.1 to 1.0) layers were grown by MBE on the (001)-oriented GaAs substrate and the (111)B-oriented GaAs substrate, and the crystal quality of grown layers were respectively evaluated. As a result, on the (001)-oriented GaAs substrate, a high-quality epitaxial layer having a perfectly smooth surface like a mirror surface was obtained, whereas an epitaxial layer grown on the (111)B-oriented GaAs substrate exhibited a hazy and rough surface, which has poor crystal quality.
Even though the semiconductor laser devices comprise a the (111)-oriented substrate, the semiconductor laser device in which the layers have poor crystal quality cannot stably and continuously emit a laser beam of light with a short wavelength at a low threshold current at room temperature.